Contents:

Correlations between trophic indicators

Nitrogen vs Phosphorus:

It should be noted:

that relative to the phosphorus
concentrations, the nitrogen/phosphorus ratio, on average, decreases
from more than 100 on the oligotrophic side to less than 10 on the
eutrophic side.
This can be interpreted as a tendency for lakes to shift from
phosphorus dependency to nitrogen dependency with increasing trophy;

that specific lakes may deviate from this rule, independent of their trophic characteristics.
Hypertrophic lakes, for example, may not be nutrient controlled at all but light-limited instead.
Deep mixing and high water replenishment rates may also reduce the effect of nutritional conditions on production.

Generally, lakes are phosphorus limited with
Total-N/Total-P ratios >15, and nitrogen limited for Total-N/Total-P
ratios <7. For ratios of Total-N/Total-P between 15 and 7, either P
or N, or both P and N could be limiting (cf. Figure-1).

The tendency for nitrogen and
phosphorus to increase in parallel makes it difficult to determine the
relative importance of the two factors in the eutrophication process.
Accordingly, it is impossible to speculate solely on the basis of
nutrient conditions found in a lake, which one of the two factors is
limiting production. This question can only be resolved by careful
analysis of all pertinent information.

Figure-1: Nitrogen vs. Phosphorus:

Ortho-phosphate-P vs. Total-Phosphorus:

With increasing trophy, both among the
ortho-phosphate/total phosphorus relationship and the mineral
nitrogen/total nitrogen relationship, the mineral component tends to
become the dominant fraction.
On average, the orthophosphate-P fraction increases from less than 20%
for Total-P concentrations of 10 mg/m3 and less, to over 45% for Total-P concentrations of 200 mg/m3
and over.
Though the trend is similar, the fractional increase in mineral
nitrogen relative to Total-N is less dramatic, increasing from 60% at
Total-N concentrations of 500 mg/m3 and less, to 70% at 5000 mg/m3 and over.

For both cases, individual lakes
may be at variance with the rule.
The importance of these trends is that, with increasing trophy a
correspondingly higher fraction of easily metabolized mineral
components become available for algal growth. In part, this greater
availability depends on the metabolic processes taking place in the
lake, but it is also due to a correspondingly greater supply of these
components in culturally eutrophied lakes, i.e., depending on the
sources causing eutrophication.

In management terms, these
findings mean that the control of sources of nitrogen and phosphorus of
high biological availability (dissolved mineral fractions) probably
have a greater effect on the reversal of eutrophication than an
unselective control, though equal in relative terms, of all sources. In
other words, priority is to be given to control of biologically readily
available components. The following Figure can be used for assessment
purposes (Kerekes, 1983).